System and method for focusing of high intensity focused ultrasound based on magnetic resonance—acoustic radiation force imaging feedback

ABSTRACT

A system and method for focusing ultrasound into a target tissue is disclosed. The method calculates a pressure field for an array of transducer elements to generate an unaberrated pressure field matrix and causes the transducer elements to apply focused ultrasound to generate a respective aberrated pressure field at a focus location in a target tissue and surrounding volume. MR-ARFI images are acquired of the focus location and surrounding volume, a magnitude of the aberrated pressure field is measured that is generated by the applied focused ultrasound at respective focus locations in the target tissue and surrounding volume based on the MR-ARFI images, an aberration for the transducer elements is determined from the pressure field matrix and the MR-ARFI images of the focus location and the surrounding volume using a magnitude least-squares method, and a corrective phase/amplitude adjustment is applied to the transducer elements based on the determined aberrations.

BACKGROUND OF THE INVENTION

Embodiments of the invention relate generally to high intensity focusedultrasound (HIFU), and more specifically, to a system and method ofemploying magnetic resonance-acoustic radiation force impulse (MR-ARFI)imaging feedback for fast and robust focusing of the HIFU.

Focused ultrasound therapy involves delivering ultrasound energy tolocalized regions of tissue from externally (non-invasive) or internally(minimally-invasive) located transducers. The amount of ultrasoundenergy delivered to tissue dictates the nature of the biologic effectproduced at that location. At high intensities with continuous exposure,ultrasound energy can generate enough heat to cause irreversible thermaldamage through coagulation. As the exposure is reduced in duty cycle toshort pulses, the mechanical energy associated with ultrasound dominatesand can be used to generate a range of bio-effects, including: vascularocclusion or hemorrhage, permeation of cells, and tissue-homogenization.

For this purpose, a piezo-ceramic transducer is placed externally to thepatient, but in close proximity to the tissue to be ablated (the“target”). The transducer converts an electronic drive signal intomechanical vibrations, resulting in the emission of acoustic waves (aprocess hereinafter referred to as “sonication”). The transducer may beshaped so that the waves converge in a focal zone. Alternatively oradditionally, the transducer may be formed of a plurality ofindividually driven transducer elements whose phases (and, optionally,amplitudes) can each be controlled independently from one another and,thus, can be set so as to result in constructive interference of theindividual acoustic waves in the focal zone. Such a “phased-array”transducer facilitates steering the focal zone to different locations byadjusting the relative phases between the transducers, and generallyprovides the higher a focus quality and resolution, the greater thenumber of transducer elements. Magnetic resonance imaging (MRI) may beutilized to visualize the focus and target in order to guide theultrasound beam.

The relative phases at which the transducer elements need to be drivento result in a focus at the target location depend on the relativelocation and orientation of the transducer surface and the target, aswell as on the dimensions and acoustic material properties (e.g., soundvelocities) of the tissue or tissues between them (i.e., the “targettissue”). Thus, to the extent the geometry and acoustic materialproperties are known, the relative phases (and, optionally, amplitudes)can be calculated. In practice, however, knowledge of these parametersis often too incomplete or imprecise to enable high-quality focusingbased on computations of the relative phases alone. For example, whenultrasound is focused into the brain to treat a tumor, the skull in theacoustic path may cause aberrations that are not readily ascertainable.In such situations, treatment is typically preceded by an auto-focusingprocedure in which, iteratively, an ultrasound focus is generated at ornear the target, the quality of the focus is measured (using, e.g.,thermal imaging or acoustic radiation force impulse (ARFI) imaging), andexperimental feedback is used to adjust the phases of the transducerelements to achieve sufficient focus quality.

The number of sonications in such an auto-focusing procedure istypically at least three times the number of individually controlledtransducer elements, and even more sonications may be needed to overcomemeasurement noise. For example, for a transducer array with 1,000elements, auto-focusing typically involves a systematic series of 3,000or more sonications to optimize the focus thereof. The auto-focusingprocedure may thus take a substantial amount of time, which may renderit impracticable or, at the least, inconvenient for a patient. Further,during the auto-focusing sonications, ultrasound energy is inevitablydeposited into the tissue at and surrounding the target, potentiallydamaging healthy tissue.

Attempts have previously been made to improve the auto-focusingprocedure using MR-ARFI to measure the quality of the focus, with suchMR-ARFI techniques being employed both to reduce the time required forperforming the procedure and to minimize the effect of pre-therapeuticsonications. For example, a previous attempt for autofocusing usingMR-ARFI is described in “Ultrasound focusing using magnetic resonanceacoustic radiation force imaging: Application to ultrasound transcranialtherapy” to Y. Hertzberg et al., Med Phys, Vol. 37, No. 6, 2010, inwhich the array of ultrasound transducers was divided into n groups(i.e., sub-arrays)—with it being assumed that all transducers in eachgroup are approximately the same. The phase/amplitude of each group oftransducers was then randomly changed until a reasonable focus wasachieved. The method described by Hertzberg et al., however, is stilltime consuming and is less systematic than is desired. That is, themethod described in Hertzberg et al. disregards MR-ARFI measurements atall voxels other than the center of the focus, such that a large numberof image acquisitions are still required.

It would therefore be desirable to provide a system and method forMR-ARFI-based autofocusing of a phased array of transducer element tocreate a high-quality ultrasound focus. It would also be desirable forsuch a system and method to reduce the number of image acquisitionsrequired to perform the autofocusing so as to reduce the time requiredfor performing the autofocusing procedure and to minimize the effect ofpre-therapeutic sonications, thereby making the MR-ARFI-basedautofocusing clinically-feasible, and possible in near-real time.

BRIEF DESCRIPTION OF THE INVENTION

In accordance with one aspect of the invention, a system for focusingultrasound into a target tissue of a subject includes a phased array ofultrasound transducer elements configured to generate an ultrasoundfocus in the target tissue, a MRI system having a plurality of gradientcoils positioned about a bore of a magnet, and an RF transceiver systemand an RF switch controlled by a pulse module to transmit RF signals toan RF coil assembly to acquire MR images, and a computer operativelyconnected to the phased array of ultrasound transducer elements and theMRI system, the computer programmed to calculate a pressure field foreach transducer element in the phased array of ultrasound transducerelements to generate an unaberrated pressure field matrix and cause thephased array of ultrasound transducer elements to apply focusedultrasound to the ROI, wherein application of focused ultrasound by eachtransducer element of the phased array of ultrasound transducer elementsgenerates a respective aberrated pressure field at a focus location inthe target tissue and in a volume surrounding the focus location. Thecomputer is further programmed to cause the MRI system to acquiremagnetic resonance acoustic radiation force impulse (MR-ARFI) images ofthe focus location and the surrounding volume, measure a magnitude ofthe aberrated pressure field generated by the applied focused ultrasoundfrom each transducer element of the phased array of ultrasoundtransducer elements at its respective focus location in the targettissue and in the volume surrounding the focus location based on theMR-ARFI images, determine an aberration for each transducer element ofthe phased array of ultrasound transducer elements from the pressurefield matrix and the MR-ARFI images of the focus location and thesurrounding volume using a magnitude least-squares method, and apply atleast one of corrective phase adjustment and a corrective amplitudeadjustment to each transducer element in the phased array of ultrasoundtransducer elements based on the determined aberration for eachrespective element.

In accordance with another aspect of the invention, a method forfocusing a phased array of ultrasound transducer elements into a targettissue using MR-ARFI includes calculating a pressure field for eachtransducer element in the phased array of ultrasound transducer elementsto generate an unaberrated pressure field matrix and causing the phasedarray of ultrasound transducer elements to apply focused ultrasound tothe ROI, wherein application of focused ultrasound by each transducerelement of the phased array of ultrasound transducer elements generatesa respective aberrated pressure field at a focus location in the targettissue and in a volume surrounding the focus location. The method alsoincludes acquiring MR-ARFI images of the focus location and thesurrounding volume, measuring a magnitude of the aberrated pressurefield generated by the applied focused ultrasound from each transducerelement of the phased array of ultrasound transducer elements at itsrespective focus location in the target tissue and in the volumesurrounding the focus location based on the MR-ARFI images, determiningan aberration for each transducer element of the phased array ofultrasound transducer elements from the pressure field matrix and theMR-ARFI images of the focus location and the surrounding volume using amagnitude least-squares method, and applying at least one of correctivephase adjustment and a corrective amplitude adjustment to eachtransducer element in the phased array of ultrasound transducer elementsbased on the determined aberration for each respective element.

In accordance with yet another aspect of the invention, a non-transitorycomputer readable storage medium is provided having stored thereon acomputer program comprising instructions that, when executed by acomputer, cause the computer to calculate a pressure field for each of aplurality of transducer elements in an ultrasound transducer to generatean unaberrated pressure field matrix, the ultrasound transducerconfigured to apply focused ultrasound to a target tissue. Theinstructions also cause the computer to cause each of the plurality oftransducer elements to apply focused ultrasound to the target tissue,with the application of focused ultrasound by each of the plurality oftransducer elements generating a respective aberrated pressure field ata focus location in the target tissue and in a volume surrounding thefocus location. The instructions further cause the computer to acquireone or more MR-ARFI images of the focus location and the surroundingvolume that each comprise one or more voxels corresponding to the focuslocation and voxels corresponding to a region surrounding the focuslocation, measure a relative tissue displacement that each transducerelement produces in the target tissue responsive to the applied focusedultrasound using the voxels in the one or more MR-ARFI imagescorresponding to the focus location and the region surrounding the focuslocation, and calculate corrective phase and amplitude adjustments foreach of the plurality of transducer elements based on the measuredrelative tissue displacement.

Various other features and advantages will be made apparent from thefollowing detailed description and the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings illustrate embodiments presently contemplated for carryingout the invention.

In the drawings:

FIG. 1 is a schematic block diagram of an exemplary MR imaging systemand focused ultrasound system for use with an embodiment of theinvention.

FIG. 2 is flowchart of a technique for autofocusing an ultrasoundtransducer array using MR-ARFI according to an embodiment of theinvention.

FIG. 3 is a graph illustrating an example of autofocusing achieved byimplementing the technique of FIG. 2.

FIG. 4 is a diagram of a phase-sensitive fast spin echo (FSE) pulsesequence according to an embodiment of the invention.

FIG. 5 is a diagram of a phase-sensitive fast spin echo (FSE) pulsesequence according to another embodiment of the invention.

FIGS. 6A-6D are diagrams of a separation and combination of even and oddechoes according to an embodiment of the invention.

FIG. 7 is a diagram of a phase-sensitive fast spin echo (FSE) pulsesequence according to another embodiment of the invention.

FIG. 8 is a table and diagram illustrating the phase of even and oddechoes for each of odd k-space lines and even k-space lines and ashifting of the image of the even echoes by half a FOV with respect tothe image of the odd echoes, according to an embodiment of theinvention.

FIG. 9 is a diagram of a restricted FOV phase-sensitive fast spin echo(FSE) pulse sequence according to another embodiment of the invention.

DETAILED DESCRIPTION

Embodiments of the invention are directed to a high intensity focusedultrasound (HIFU) system and associated magnetic resonance (MR) imagingsystem and that employs magnetic resonance-acoustic radiation forceimpulse (MR-ARFI) imaging feedback for fast and robust focusing of theHIFU.

Referring to FIG. 1, the major components of an MR imaging system 10 anda focused ultrasound system 70 (i.e., HIFU system) are shownincorporating an embodiment of the invention. With regard first to theMR imaging system 10, the operation of the system is controlled forcertain functions from an operator console 12 which in this exampleincludes a keyboard or other input device 13, a control panel 14, and adisplay screen 16. The console 12 communicates through a link 18 with aseparate computer system 20 that enables an operator to control theproduction and display of images on the display screen 16. The computersystem 20 includes a number of modules which communicate with each otherthrough a backplane 20 a. These modules include an image processormodule 22, a CPU module 24 and a memory module 26, known in the art as aframe buffer for storing image data arrays. The computer system 20communicates with a separate system control 32 through a high speedserial link 34. The input device 13 can include a mouse, joystick,keyboard, track ball, touch activated screen, light wand, voice control,card reader, push-button, or any similar or equivalent input device, andmay be used for interactive geometry prescription.

The system control 32 includes a set of modules connected together by abackplane 32 a. These include a CPU module 36 and a pulse generatormodule 38 which connects to the operator console 12 through a seriallink 40. It is through link 40 that the system control 32 receivescommands from the operator to indicate the scan sequence that is to beperformed. The pulse generator module 38 operates the system componentsto carry out the desired scan sequence and produces data which indicatesthe timing, strength and shape of the RF pulses produced, and the timingand length of the data acquisition window. The pulse generator module 38connects to a set of gradient amplifiers 42, to indicate the timing andshape of the gradient pulses that are produced during the scan. Thepulse generator module 38 can also receive patient data from aphysiological acquisition controller 44 that receives signals from anumber of different sensors connected to the patient, such as ECGsignals from electrodes attached to the patient. And finally, the pulsegenerator module 38 connects to a scan room interface circuit 46 whichreceives signals from various sensors associated with the condition ofthe patient and the magnet system. It is also through the scan roominterface circuit 46 that a patient positioning system 48 receivescommands to move the patient to the desired position for the scan.

The gradient waveforms produced by the pulse generator module 38 areapplied to the gradient amplifier system 42 having Gx, Gy, and Gzamplifiers. Each gradient amplifier excites a corresponding physicalgradient coil in a gradient coil assembly generally designated 50 toproduce the magnetic field gradients used for spatially encodingacquired signals. The gradient coil assembly 50 forms part of aresonance assembly 52 which includes a polarizing magnet 54 and awhole-body RF coil 56. A transceiver module 58 in the system control 32produces pulses which are amplified by an RF amplifier 60 and coupled tothe RF coil 56 by a transmit/receive switch 62. The resulting signalsemitted by the excited nuclei in the patient may be sensed by the sameRF coil 56 and coupled through the transmit/receive switch 62 to apreamplifier 64. The amplified MR signals are demodulated, filtered, anddigitized in the receiver section of the transceiver 58. Thetransmit/receive switch 62 is controlled by a signal from the pulsegenerator module 38 to electrically connect the RF amplifier 60 to thecoil 56 during the transmit mode and to connect the preamplifier 64 tothe coil 56 during the receive mode. The transmit/receive switch 62 canalso enable a separate RF coil (for example, a surface coil) to be usedin either the transmit or receive mode.

The MR signals picked up by the RF coil 56 are digitized by thetransceiver module 58 and transferred to a memory module 66 in thesystem control 32. A scan is complete when an array of raw k-space datahas been acquired in the memory module 66. This raw k-space data isrearranged into separate k-space data arrays for each image to bereconstructed, and each of these is input to an array processor 68 whichoperates to Fourier transform the data into an array of image data. Thisimage data is conveyed through the serial link 34 to the computer system20 where it is stored in memory. In response to commands received fromthe operator console 12 or as otherwise directed by the system software,this image data may be archived in long term storage or it may befurther processed by the image processor 22 and conveyed to the operatorconsole 12 and presented on the display 16.

With regard to the focused ultrasound system 70, the system ispositioned, at least in part, adjacent to (or within the bore of) MRIsystem 10. The focused ultrasound system includes an ultrasoundtransducer 72, which is disposed near a subject of interest 74 anddirected towards a target 76 in a region of interest (“ROI”) inside thepatient. The transducer 72 may comprise a one- or two-dimensional array(i.e., a row or a matrix) of individually controllable transducerelements 78. In other embodiments, the transducer elements 78 may bearranged in a non-coordinated fashion, i.e., they need not be spacedregularly or arranged in a regular pattern. The transducer may have acurved (e.g., spherical or parabolic) shape, as illustrated, or mayinclude one or more planar or otherwise shaped sections. Its dimensionsmay vary, depending on the application, between millimeters and tens ofcentimeters. The transducer elements 78 may be piezoelectric ceramicelements. Piezo-composite materials, or generally any materials capableof converting electrical energy to acoustic energy, may also be used. Todamp the mechanical coupling between the elements 78, they may bemounted on the housing using silicone rubber or any other suitabledamping material.

The transducer elements 78 are separately controllable, i.e., they areeach capable of emitting ultrasound waves at amplitudes and/or phasesthat are independent of the amplitudes and/or phases of the othertransducers. A transducer controller 80 in communication with the arrayserves to drive the transducer elements 78. For n transducer elements78, the controller 80 may contain n control circuits, each comprising anamplifier and a phase delay circuit and driving one of the transducerelements. The controller 80 may split a radio frequency (RF) inputsignal, typically in the range from 0.1 MHz to 4 MHz, to provide nchannels for the n control circuits. The controller may be configured todrive the individual transducer elements 78 at the same frequency, butat different phases and different amplitudes so that they collectivelyproduce a focused ultrasound beam. The controller 80 may also includeadditional circuitry and switches that allow subsets of the transducerelements to be grouped into sub-arrays, and the elements within onesub-array to be driven at the same amplitude and phase.

The controller 80 desirably provides computational functionality, whichmay be implemented in software, hardware, firmware, hardwiring, or anycombination thereof, to compute the required phases and amplitudes for adesired focus location. For example, the controller 80 may receive dataindicative of the desired focus location (i.e., the target) relative tothe ultrasound transducer, and account for the respective distancesbetween each transducer element and the target, and the associatedtravel times of the acoustic waves that originate at the varioustransducer elements, in computing the phases. If the sum of thetransducer element phase and the phase acquired between the transducerelement and the target is the same for all elements, the waves from thedifferent transducer elements constructively interfere at the target.

In general, the controller 80 may include several separable apparatus,such as a frequency generator, a beamformer containing the amplifier andphase delay circuitry, and a computer (e.g., a general-purpose computer)performing the computations and communicating the phases and amplitudesfor the individual transducer elements 78 to the beamformer(s).Additionally, it is recognized that the controller 80 is operablyconnected to the computer 20 of MRI system 10, such that the controller80 can control transducer 72 to time an application of focusedultrasound to a target tissue to coincide with generation/application ofa pulse sequence from MRI system 10, to provide for MR-ARFIvisualization of the applied ultrasound, as explained in greater detailbelow.

In performing of a focused ultrasound procedure for treating the target76 in the ROI, the focus of the ultrasound can be visualized usingMR-ARFI so as to confirm the location and measure the quality of thefocus. In MR-ARFI, a transducer is driven so as to focus an ultrasoundwave pulse into the body at or near the target. The ultrasound waveexerts acoustic radiation pressure onto the material along its path. Atthe focus, where the waves converge, this pressure is highest, resultingin a temporary local displacement of the material from equilibrium inthe longitudinal direction and/or in shear waves that propagate radiallyaway from the focus—with the displacement varying between about −1 μmand 5 μm depending on the intensity of the acoustic field. Thus, theultrasound pressure creates a displacement field that directly reflectsthe acoustic field. The displacement field may be visualized by applyingtransient-motion or displacement-sensitizing magnetic field gradients tothe imaging region by gradient coils, such as gradient coil assembly 50in MR imaging system 10. When the ultrasound pulse is applied in thepresence of such gradients, the resulting displacement is directlyencoded into the phase of the MR response signal. For example, thegradient coils and transducer may be configured such that the ultrasoundpulse pushes material near the focus towards regions of the magneticfield with higher field strengths. In response to the resulting changein the magnetic field, the phase of the MR response signal changesproportionally, thereby encoding in the signal the displacement causedby the ultrasound radiation pressure.

MR-ARFI may be used to “auto-focus” an ultrasound beam (i.e., toiteratively improve the focus quality of a pre-focused beam based onexperimental feedback) in advance of the therapeutic application ofultrasound. Consider, for example, the treatment of a brain tumor withultrasound. A transducer for such an application is usually large; itmay surround a wide area of the skull and comprise a large number ofelements (e.g., 1000). In preparation for treatment, the transducer istypically placed in a stable position relative to the patient's head,and the transducer elements are then activated at relative phases basedon the sonication geometry (which generally includes the relativeposition and orientation of transducer and the target tissue, as well asthe target location). Optionally, phase corrections may be applied tothe transducer elements to compensate for tissue aberrations, which aremostly caused by the intervening skull tissue and which may varysignificantly with location. The phase corrections may be computed basedon skull-imaging data obtained through MR imaging, which provideestimates of the local skull bone thickness and density. Often, suchcomputational correction for skull-based aberrations results in anoticeable, yet insufficient improvement of the focus quality. The focusmay be optimized with an auto-focusing procedure, in which low-energyultrasound is focused at (or near) the target, and a quantity correlatedto the focus quality (e.g., the peak displacement caused by radiationforce) is measured.

According to embodiments of the invention, auto-focusing is performedusing MR-ARFI, with the ultrasound field being determined by measuringthe relative tissue displacement that each transducer element produces.To address target tissue aberrations caused by bone and/or otherintervening tissue(s), an autofocusing method is employed in embodimentsof the present invention that uses the entire MR-ARFI image (i.e.,analyzes voxels at the target/focus location (center of the focus) andvoxels of a surrounding volume, such an entirety of a surroundingtissue) to determine the aberrations, with a corrective phase/amplitudefor each transducer being calculated based on analysis of the MR-ARFIimage. By using the entire MR-ARFI image to determine the aberrationsrather than just the voxels at the center of the focus in the MR-ARFIimage (as is done in some existing prior art techniques), the number ofimage acquisitions required to perform the autofocusing is dramaticallyreduced, thereby making MR-ARFI-based autofocusing clinically-feasible,and possible in near-real time.

Referring now to FIG. 2, a technique 82 for focusing a transducer arrayusing MR-ARFI is illustrated according to an embodiment of theinvention. As shown in FIG. 2, technique 82 begins by calibrating orbuilding a pressure field matrix P at STEP 84 that is produced bytransducer elements without aberration (i.e., unaberrated pressurefield). The pressure field matrix P for the transducer elements isconstructed/built offline (pre-calibrated or pre-computed) using MR-ARFIimages of a phantom or tissue sample, for example. Alternatively, thepressure field matrix P could also be constructed using numericalsimulations in various tissue types, and the appropriate simulationcould be chosen in a manner tailored to the target tissue duringfocusing.

Technique 82 continues by measuring the magnitudes of aberrated pressurefields using MR-ARFI at STEP 86—with the pressure fields being aberratedbased on the presence of bone or other intervening materials between thetransducer and the target focus point. As set forth above, in MR-ARFI,the displacement field resulting from the acoustic radiation pressuregenerated by an ultrasound wave exerts may be visualized by applyingtransient-motion or displacement-sensitizing magnetic field gradients.When the ultrasound pulse is applied in the presence of such gradients,the resulting displacement is directly encoded into the phase of the MRresponse signal. For example, the gradient coils and transducer may beconfigured such that the ultrasound pulse pushes material near the focustowards regions of the magnetic field with higher field strengths. Inresponse to the resulting change in the magnetic field, the phase of theMR response signal changes proportionally, thereby encoding in thesignal the displacement caused by the ultrasound radiation pressure.

Upon measuring of the magnitudes of the aberrated pressure fields usingMR-ARFI, the technique continues at STEP 88 where a determination of theaberrations is performed. In determining the aberrations, it is assumedthat the HIFU pressure field in and around the focus can be expressed asthe complex sum of the fields produced by the transducer elements, whichare phase-shifted and attenuated by aberrations. In the absence ofaberrations, the total HIFU pressure field is:p({right arrow over (r)})=Σ_(n=1) ^(N) p _(n)({right arrow over(r)})  (1),where p_(n)({right arrow over (r)}) is the complex-valued field producedby transducer element n, in the absence of aberrations. To a goodapproximation, aberrations and attenuations such as those produced bythe skull can be expressed as a sum of phase-shifted and attenuatedfields:{tilde over (p)}({right arrow over (r)})=Σ_(n=1) ^(N) a _(n) p_(n)({right arrow over (r)})  (2),where the a_(n) are complex-valued attenuation and phase aberrations.

If the transducer elements' pressure fields p_(n)({right arrow over(r)}) and the total aberrated field {tilde over (p)}({right arrow over(r)}) could be measured directly, then the aberrations could bedetermined by solving a system of linear equations. However, the MR-ARFIimage is proportional to |{tilde over (p)}({right arrow over (r)})|²,with a constant of proportionality that is tissue-dependent—and thus thephase of {tilde over (p)}({right arrow over (r)}) cannot be measured. Assuch, in STEP 88, a magnitude least-squares regression method is used tosolve for the aberrations, given MR-ARFI images of |{tilde over(p)}({right arrow over (r)})|² and a matrix of the individual elements'pressure fields {tilde over (p)}({right arrow over (r)}). According toembodiments of the invention, various regressions could be implementedto solve for the aberrations, including a sum-of-squared errors, amaximum error norm, or a sum-of-absolute errors regression, for example.According to an exemplary embodiment, the magnitude least-squares methodminimizes the following objective/cost function to solve for thelength-n aberration vector a:Ψ(a)=Σ_(l=1) ^(L) ∥|{tilde over (p)} _(l) |−Pdiag(w _(n) ^(l))a∥ _(Wl)^(Z) +R(a)  (3),where the first term is formed by discretizing Eq. (2) and measuring thel₂ distance between right and left hand sides, summed across more thanone image voxel. The variable l in Eq. (3) indexes the possibly multipleMR-ARFI acquisitions, each acquired with a possibly unique set oftransducer element amplitudes and phase shifts w^(l) that togetherproduce L logical transducer elements. For example, the vectors w^(l)could be Hadamard vectors of 1's and −1's. The pressure field matrix P(which has a row dimension equal to the number of voxels in the ARFIimage, and column dimension equal to N) is known and provided from STEP84, where P was constructed offline. Relative phases of the elements'ARFI images can be measured using an interference method, such as in: B.Larrat, M. Perrot, G. Montaldo, M. Fink, and M. Tanter, MR-guidedadaptive focusing of ultrasound, IEEE Trans Ultrason Ferroelectr FreqControl, 57(8):1734-1747, 2010.

It is noted that the array's physical elements could also be groupedinto logical subsets, and that those subsets could be focused using thismethod. Further, in Eq. (3), the error weighting matrix W_(l) can beused to account for variations in SNR and noise correlations betweenspatial locations, for example by setting W_(l)=diag(|I_(l)({right arrowover (r)}_(j))|), where I_(l) is one of the two complex-valued MR imagesfrom which {tilde over (p)}_(l) was derived, and j indexes spatiallocations. Finally, the regularization function R(a) could beconstructed to, e.g., enforce smoothness of the aberrations betweentransducer elements, low integrated squared magnitude across theaberrations, or low total variation between adjacent aberrations.

Once the aberration vector a for each transducer element is computed atSTEP 88 by minimizing the cost function in Eq. (3), the array is focusedat STEP 90 by applying magnitude and/or phase corrections to eachtransducer element 78. To apply magnitude and phase corrections, eachelement's input signal is multiplied by a magnitude of its respectiveaberration vector |a_(n) ⁻¹|, thereby imparting both a magnitude andphase correction. Alternatively, a phase-only correction can be achievedby adding angles of the respective aberration vectors ∠a_(n) ⁻¹ to thephase of each transducer element's input signal.

Technique 82 thus provides an MR-ARFI-based autofocusing method thatrequires only a small number of image acquisitions, compared to thethousands required by existing methods, so as to enable autofocusing ina clinically-feasible time. The technique can be applied to focusingHIFU through any bones (including the skull), to focusing throughinhomogeneous tissue (e.g., scars and fat between the transducer and thetarget focus), or to detecting the existence and position of ribs infront of the transducer, in order to switch off elements directly infront of the ribs.

An example of autofocusing achieved by implementing technique 82 isillustrated in FIG. 3, where autofocusing was performed according totechnique 82 after random phase aberrations were applied to thetransducer. Using L=4 Hadamard encodings corresponding to 8 total MRimage acquisitions, the technique restored the pressure field from anunfocused state, indicated at 92, to its focused state, indicated at 94.The eight (8) total MR image acquisitions is compared to a conventionalmethod that required 6,144 total MR image acquisitions (i.e., 6 pertransducer element, 6*1024).

It is recognized that, in implementing the MR-ARFI-based autofocusingmethod of the present invention, it is desirable that the MR pulsesequence used for image acquisition provide images that are free ofdistortions and signal loss induced by B0 and B1 inhomogeneity. That is,it is recognized that for certain HIFU applications, such as brain tumortreatment for example, where significant instrumentation is presentinside the magnet bore of the MR system—e.g., helmet around theskull—significant B0 and B1 inhomogeneity is induced. In a brainapplication where the accuracy of ablation must be very high (<1 mm),such B0 and B1 inhomogeneity is highly undesirable as it can negativelyaffect the accuracy of the ablation. As such, it is desirable to providea phase sensitive MR pulse sequence for image acquisition that willimprove image quality in ARFI, with the use of such sequences inconjunction with the proposed MR-ARFI autofocusing technique 82 (andalgorithm implemented therein) providing improved robustness and speedin the ultrasound autofocusing. While the MR pulse sequences describedbelow are discussed with respect to use with ARFI, it is recognized thatthe pulse sequences are useful for a number of different MR imagingtechniques that acquire phase sensitive data, such as temperature(PRF-shift) maps and Block-Siegert B1 maps, for example. Therefore, thepulse sequences described below are understood not to be limited for useonly for ARFI applications.

With respect to designing a phase sensitive MR pulse sequence, the ideaof using a Fast Spin Echo (FSE) echo train to measure phase wasdescribed in M. W. Vogel et. al., J. Mag. Res. Imag. Vol 18, p. 507-512(2003), using a sophisticated phase modulation scheme. However, thismethod was not used in practice due to the need of a calibration whichis sensitive to system imperfections. The separation of even and oddechoes to removed artifacts was introduced by Y. Zur and S. Stokar, J.Mag. Res. Vol 71, p. 212-228 (1987), using phase cycling and/or echoshifting. This echo time shift method was later used by DG Norris et.al. (Mag. Res. Med. Vol 27, p. 142, (1992)), F. Schick (Mag. Res. Med.Vol 38, p. 638 (1997)), and C Williams et. al. (Mag. Res. Med. Vol 41,p. 734 (1999)). Embodiments of the present invention separate even andodd echoes and add them coherently to enhance signal-to-noise ratio(SNR)—by 1.4—and eliminate echo oscillations while preserving the phaseprior to the first refocusing RF pulse.

According to an exemplary embodiment, a phase sensitive FSE method isprovided that measures the phase, φ₀, induced by ARFI at each location,while being free of distortions and signal loss induced by B0inhomogeneity. The phase φ₀ (shown in FIG. 5 and Eq. (4)) is measuredwith the signals from an FSE echo train. The echo train consists of alarge number, N, of echoes separated by a short echo time, such that afull image can be reconstructed using a single or a small number of echotrains. The phase of each voxel in the reconstructed image isproportional to φ₀. This technique is based on the fact that the signalafter each 180 pulse is broken into two echoes, with the phasedifference between them being proportional to the phase prior to thefirst 180 pulse. Therefore, by separating the echoes, the phase prior tothe FSE train—which can be caused by temperature change, ARFI, BlochSiegert B1 shift, etc.—can be measured. By a proper combination of thetwo echoes, the signal becomes smooth and the signal-to-noise ratio(SNR) improves by, for example, a factor of 1.4. Additionally, sinceduring data acquisition the signal is fully refocused, there are nospatial shifts or signal loss due to B0 inhomogeneity.

In FIG. 4, a phase-sensitive FSE pulse sequence 100 is shown accordingto one embodiment. The FSE sequence 100 includes a preparation segment102 and a sequence of N refocusing pulses applied during identical timesegments, referred to below as refocusing segments 104. To distinguishbetween RF phases and magnetization (signal) phases, we use the letter φfor RF phases and φ for magnetization phases. The RF phase duringrefocusing segment k is φ_(k), k=1 to N. The phase of the RF pulse inthe preparation segment (“90°” RF pulse) is φ₀. φ_(i) is the phaseaccumulated during the preparation segment 102, and φ₀ is the phaseinduced during the preparation segment 102 by thermal, ARFI, fMRI etc.,that we want to measure. φ_(i) is given by:

$\begin{matrix}{\varphi_{i} = {\phi_{0} + \frac{\pi}{2} + {\varphi_{0}.}}} & (4)\end{matrix}$

Assuming that all refocusing segments 104 are identical and that thephases of all refocusing pulses are zero, the Meiboom-Gill condition isfulfilled if φ_(i) is zero. If φ_(i)≠0, the signal in each segment 104breaks into two echoes: 1) an “Even” echo 106 which is generated bymagnetization pathways with an even number of phase inversions; 2) an“Odd” echo 108 which is generated by magnetization pathways with an oddnumber of phase inversions, as is shown in FIG. 5. The phase of the evenechoes 106 is proportional to φ_(i) because the number of inversions iseven. The phase of the odd echoes 108 is proportional to −φ_(i) becausethe number of inversions is odd. The goal is to find φ_(i) and φ₀ (Eq.(4)) from the preparation segment 102. This is done by separating theeven and the odd echoes 106, 108 and calculating the phase differencebetween them.

The even and odd echoes 106, 108 can be separated by 1) a two-shot phasecycling, or 2) by separating them in time in each refocusing segment(i.e., time shifting) and using a single shot, as explained below. Inseparating the echoes, the signal of the even echoes (with phase φ_(i))is added to the complex conjugate of the odd echoes signal (with phase−φ_(i)) and a smooth signal is obtained with similar magnitude to thefully refocused FSE but with phase φ_(i).

In separating the even and odd echoes 106, 108 via a two-shot phasecycling, two shots with different RF phases are run. The generalrelation between the RF phases in these two shots required to separatethe even and odd echoes is given by Y. Zur and S. Stokar, J. Mag. Res.Vol 71, p. 212-228 (1987). The simplest combination according to:

$\begin{matrix}\begin{matrix}{{shot}\mspace{14mu} 1} & {\phi_{0} = 0} & {\phi_{k} = 0} & {{{for}\mspace{14mu}{all}\mspace{14mu} k} = {1\mspace{14mu}{to}\mspace{14mu} N}}\end{matrix} & ( {5a} ) \\\begin{matrix}{{shot}\mspace{14mu} 2} & {\phi_{0} = {- \frac{\pi}{2}}} & {\phi_{k} = 0} & {{{for}\mspace{14mu}{all}\mspace{14mu} k} = {1\mspace{14mu}{to}\mspace{14mu}{N.}}}\end{matrix} & ( {5b} )\end{matrix}$The phase of the echoes in each shot is the same during all N refocusingsegments.

In shot 1:phase of even echoes=π/2+φ₀ and phase of odd echoes=−π/2−φ₀  (6a).

In shot 2:phase of even echoes=φ₀ and phase of odd echoes=−φ₀  (6b).

The separation of the even echo S_(even) and the odd echo S_(odd) usingthe signals S₁ and S₂ of shots 1 and 2 is shown in FIGS. 6A-6D, withFIG. 6A showing even and odd echoes from the signals S₁ and S₂ of shots1 and 2, FIG. 6B showing S₁ rotated by 90°—such that the even echoes e1,e2 are opposite to each other and odd echoes o1, o2 are in phase, FIG.6C showing the even and odd echoes being separated by subtraction andaddition of iS₁ and S₂, and FIG. 6D showing the even echo S_(even) andthe complex conjugate of the odd echo S_(odd) being aligned, generatingthe signal S with phase φ₀. From FIGS. 6A-6D:

$\begin{matrix}{{S_{odd} = \frac{{iS}_{1} + S_{2}}{2}};{S_{even} = {\frac{{iS}_{1} - S_{2}}{2}.}}} & ( {7a} )\end{matrix}$

The final image S with phase φ₀ is obtained byS=S _(even) +S* _(odd)  (7b).

In practice the phases of the even (odd) echoes may have error termφ_(E1) (φ_(E2)) respectively due to misplaced sampling window, eddycurrents etc.

In shot 1:phase of even echoes=π/2+φ₀+φ_(E1) andphase of odd echoes=−π/2−φ₀+φ_(E2)  (8a).

In shot 2:phase of even echoes=φ₀+φ_(E1) andphase of odd echoes=−φ₀+φ_(E2)  (8b).

The even and odd echoes are separated using Eq. (7a), but the phases areφ₀+φ_(E1) and −φ₀+φ_(E2). For any phase sensitive scan (thermal, ARFI,b₁, etc.), a reference scan is acquired prior to heating, where φ₀=0, toremove irrelevant error phase such as φ_(E1) and φ_(E2). After removingthe error phase, the phase of the even and odd echoes is given by (6a)and (6b) and the echoes are combined as in (7b).

It is possible to separate even and odd echoes 106, 108 via timeshifting, by adding a read gradient 110 of duration τ sec in thepreparation segment 102, the even (odd) echo in each refocusing segment104 shifts to the left (right) by τ sec, indicated at 112, as shown inFIG. 7. Each echo 106, 108 is sampled in a separate sampling window, andthe phase difference is used to measure φ₀. The even and the odd echoes106, 108 are combined to a signal S as in Eq. (7b), where S variessmoothly in time and has the same amplitude as the conventional singleshot FSE signal.

While only a single shot is required to separate the echoes 106, 108,the drawback of the method is the need to use two sampling windows ineach refocusing segment 104 (FIG. 5), so the time distance between twoadjacent refocusing pulses (esp) is doubled, resulting in contrast andsignal to noise reduction. In addition, the signal of each echo group(even and odd) is very oscillatory. A coherent addition is verysensitive to system imperfection and not practical in practice, asdescribed in C Williams et al. Mag. Res. Med. Vol. 41, p. 734 (1999).

According to one embodiment of the invention, the number of k spacelines in each echo train may be reduced using parallel imaging. In thefirst shot, the odd k-space lines (1, 3, 5, . . . ) are acquired, and inthe second shot, even lines (2, 4, 6, . . . ) are acquired. In thiscase, the unacquired lines are calculated using parallel imaging withacceleration R=2 to separate even and odd echoes.

As shown in FIG. 8, if the RF phases of Eq. (5) are used, the phase ofthe even echo of iS₁ (i=sqrt(−1)) and S2 (Eq. [3]) are φ₀+π and φ₀respectively and the phase of the odd echo of iS₁ and S₂ are −φ₀ and −φ₀respectively. During the first FSE shot (where odd k-space lines areacquired) we use the RF phases of Eq. (5a) and during the second shot(even k-space lines) the phases of Eq. (5b). Therefore the phase of theeven echo 106 for even and odd k-space lines is φ₀+π, and φ₀respectively. The phase of the odd echo 108 for even and odd k-spacelines is −φ₀, and −φ₀ respectively. Hence, after Fourier transform, theimage of the even echoes 106 (with phase φ₀) is shifted by half FOV withrespect to the image of the odd echoes 108 (with phase −φ₀) along thephase encode direction, as shown in FIG. 8. Due to this shift thegeometry factor (g factor) of the images decreases significantly, asdescribed in F. A. Breuer et. al., Mag. Res. Med, Vol. 53, p. 684-691(2005), so the images of the even and odd echoes can be faithfullyreconstructed with R=2 parallel imaging.

In implementing embodiments of the invention, it is recognized that inmost cases the area of interest for thermal and ARFI applications islimited, e.g. 5 to 10 cm. Therefore, for a given spatial resolution, amuch lower number of phase encoding lines are required, provided thatthe signal from spins outside the limited FOV of 5 to 10 cm can besuppressed. This is very useful for the ssFSE scan, because a much lowernumber of echoes need to be acquired in a single shot.

According to one embodiment of the invention, a technique forsuppressing a signal from outside the restricted FOV is provided inwhich different slice-select gradients are applied during thepreparation segment RF pulse (“90°”) and during the refocusing RFpulses. Application of the slice-select gradients at these times ensuresthat only spins at the cross-section of the slices excited by both RFpulses are detected. The drawback of this technique is that themulti-slice capability of ssFSE is lost, since the RF pulses excitealong two orthogonal directions.

Referring to FIG. 9, a pulse sequence 120 for suppressing a signal fromoutside the restricted FOV is provided according to one embodiment ofthe invention. In the sequence, slice select variable rate selectiveexcitation (VERSE) gradients 122 are applied in the phase direction, sothe FOV along the phase is restricted to about 10 cm—with the sliceselect VERSE gradients being applied prior to the ssFSE preparationsegment and ssFSE train. Additionally, the sequence 120 applies a trainof up to three multi-band quadratic phase RF pulses 124 that saturatethe spins above and below the restricted FOV. It can be shown that incase of b₁ inhomogeneity, a train of three RF pulses achieves perfectsuppression even with b₁ variation of 100%. However, in most cases asingle RF pulse is sufficient. The quadratic phase RF pulses 124 are lowpower with very high time-bandwidth product. In this way, robust signalsuppression is obtained without losing the multi-slice capability.

Therefore, according to embodiments of the invention shown and describedin FIGS. 4-9, complimentary FSE and ssFSE sequences are provided thatwill improve image quality in ARFI and other phase-based MR imagingtechniques. Use of these sequences in conjunction with the proposedMR-ARFI autofocusing algorithm in technique 82 will dramatically improvethe robustness and speed of ultrasound autofocusing.

Beneficially, embodiments of the invention thus provide an MR-ARFI-basedautofocusing method that requires only a small number of imageacquisitions, compared to the thousands required by current methods, soas to enable autofocusing in a clinically-feasible time. The methodcould be applied to focusing HIFU through any bones including the skull,to focusing through inhomogeneous tissue (e.g., scars and fat betweenthe transducer and the target focus), or to detecting the existence andposition of ribs in front of the transducer, in order to switch offelements directly in front of the ribs. The technique enables real-timeacquisition of ARFI images that are free of spatial distortion andsignal loss.

A technical contribution of the invention is that it provides a computerimplemented technique for MR-ARFI-based autofocusing of focusedultrasound that requires only a small number of image acquisitions.

One skilled in the art will appreciate that embodiments of the inventionmay be interfaced to and controlled by a computer readable storagemedium having stored thereon a computer program. The computer readablestorage medium includes a plurality of components such as one or more ofelectronic components, hardware components, and/or computer softwarecomponents. These components may include one or more computer readablestorage media that generally stores instructions such as software,firmware and/or assembly language for performing one or more portions ofone or more implementations or embodiments of a sequence. These computerreadable storage media are generally non-transitory and/or tangible.Examples of such a computer readable storage medium include a recordabledata storage medium of a computer and/or storage device. The computerreadable storage media may employ, for example, one or more of amagnetic, electrical, optical, biological, and/or atomic data storagemedium. Further, such media may take the form of, for example, floppydisks, magnetic tapes, CD-ROMs, DVD-ROMs, hard disk drives, and/orelectronic memory. Other forms of non-transitory and/or tangiblecomputer readable storage media not list may be employed withembodiments of the invention.

A number of such components can be combined or divided in animplementation of a system. Further, such components may include a setand/or series of computer instructions written in or implemented withany of a number of programming languages, as will be appreciated bythose skilled in the art. In addition, other forms of computer readablemedia such as a carrier wave may be employed to embody a computer datasignal representing a sequence of instructions that when executed by oneor more computers causes the one or more computers to perform one ormore portions of one or more implementations or embodiments of asequence.

Therefore, according to one embodiment of the invention, a system forfocusing ultrasound into a target tissue of a subject includes a phasedarray of ultrasound transducer elements configured to generate anultrasound focus in the target tissue, a MRI system having a pluralityof gradient coils positioned about a bore of a magnet, and an RFtransceiver system and an RF switch controlled by a pulse module totransmit RF signals to an RF coil assembly to acquire MR images, and acomputer operatively connected to the phased array of ultrasoundtransducer elements and the MRI system, the computer programmed tocalculate a pressure field for each transducer element in the phasedarray of ultrasound transducer elements to generate an unaberratedpressure field matrix and cause the phased array of ultrasoundtransducer elements to apply focused ultrasound to the ROI, whereinapplication of focused ultrasound by each transducer element of thephased array of ultrasound transducer elements generates a respectiveaberrated pressure field at a focus location in the target tissue and ina volume surrounding the focus location. The computer is furtherprogrammed to cause the MRI system to acquire magnetic resonanceacoustic radiation force impulse (MR-ARFI) images of the focus locationand the surrounding volume, measure a magnitude of the aberratedpressure field generated by the applied focused ultrasound from eachtransducer element of the phased array of ultrasound transducer elementsat its respective focus location in the target tissue and in the volumesurrounding the focus location based on the MR-ARFI images, determine anaberration for each transducer element of the phased array of ultrasoundtransducer elements from the pressure field matrix and the MR-ARFIimages of the focus location and the surrounding volume using amagnitude least-squares method, and apply at least one of correctivephase adjustment and a corrective amplitude adjustment to eachtransducer element in the phased array of ultrasound transducer elementsbased on the determined aberration for each respective element.

According to another embodiment of the invention, a method for focusinga phased array of ultrasound transducer elements into a target tissueusing MR-ARFI includes calculating a pressure field for each transducerelement in the phased array of ultrasound transducer elements togenerate an unaberrated pressure field matrix and causing the phasedarray of ultrasound transducer elements to apply focused ultrasound tothe ROI, wherein application of focused ultrasound by each transducerelement of the phased array of ultrasound transducer elements generatesa respective aberrated pressure field at a focus location in the targettissue and in a volume surrounding the focus location. The method alsoincludes acquiring MR-ARFI images of the focus location and thesurrounding volume, measuring a magnitude of the aberrated pressurefield generated by the applied focused ultrasound from each transducerelement of the phased array of ultrasound transducer elements at itsrespective focus location in the target tissue and in the volumesurrounding the focus location based on the MR-ARFI images, determiningan aberration for each transducer element of the phased array ofultrasound transducer elements from the pressure field matrix and theMR-ARFI images of the focus location and the surrounding volume using amagnitude least-squares method, and applying at least one of correctivephase adjustment and a corrective amplitude adjustment to eachtransducer element in the phased array of ultrasound transducer elementsbased on the determined aberration for each respective element.

According to yet another embodiment of the invention, a non-transitorycomputer readable storage medium is provided having stored thereon acomputer program comprising instructions that, when executed by acomputer, cause the computer to calculate a pressure field for each of aplurality of transducer elements in an ultrasound transducer to generatean unaberrated pressure field matrix, the ultrasound transducerconfigured to apply focused ultrasound to a target tissue. Theinstructions also cause the computer to cause each of the plurality oftransducer elements to apply focused ultrasound to the target tissue,with the application of focused ultrasound by each of the plurality oftransducer elements generating a respective aberrated pressure field ata focus location in the target tissue and in a volume surrounding thefocus location. The instructions further cause the computer to acquireone or more MR-ARFI images of the focus location and the surroundingvolume that each comprise one or more voxels corresponding to the focuslocation and voxels corresponding to a region surrounding the focuslocation, measure a relative tissue displacement that each transducerelement produces in the target tissue responsive to the applied focusedultrasound using the voxels in the one or more MR-ARFI imagescorresponding to the focus location and the region surrounding the focuslocation, and calculate corrective phase and amplitude adjustments foreach of the plurality of transducer elements based on the measuredrelative tissue displacement.

This written description uses examples to disclose the invention,including the best mode, and also to enable any person skilled in theart to practice the invention, including making and using any devices orsystems and performing any incorporated methods. The patentable scope ofthe invention is defined by the claims, and may include other examplesthat occur to those skilled in the art. Such other examples are intendedto be within the scope of the claims if they have structural elementsthat do not differ from the literal language of the claims, or if theyinclude equivalent structural elements with insubstantial differencesfrom the literal languages of the claims.

What is claimed is:
 1. A system for focusing ultrasound into a targettissue of a subject, the system comprising: a phased array of ultrasoundtransducer elements configured to generate an ultrasound focus in thetarget tissue; a magnetic resonance imaging (MRI) system having aplurality of gradient coils positioned about a bore of a magnet, and anRF transceiver system and an RF switch controlled by a pulse module totransmit RF signals to an RF coil assembly to acquire MR images; and acomputer operatively connected to the phased array of ultrasoundtransducer elements and the MRI system, the computer programmed to:calculate a pressure field for each transducer element in the phasedarray of ultrasound transducer elements to generate an unaberratedpressure field matrix; cause the phased array of ultrasound transducerelements to apply focused ultrasound to the target tissue, whereinapplication of focused ultrasound by each transducer element of thephased array of ultrasound transducer elements generates a respectiveaberrated pressure field at a focus location in the target tissue and ina volume surrounding the focus location; cause the MRI system to acquiremagnetic resonance acoustic radiation force impulse (MR-ARFI) images ofthe focus location and the surrounding volume; based on the MR-ARFIimages, measure a magnitude of the aberrated pressure field generated bythe applied focused ultrasound from each transducer element of thephased array of ultrasound transducer elements at its respective focuslocation in the target tissue and in the volume surrounding the focuslocation; determine an aberration for each transducer element of thephased array of ultrasound transducer elements from the pressure fieldmatrix and the MR-ARFI images of the focus location and the surroundingvolume using a magnitude least-squares method; and apply at least one ofcorrective phase adjustment and a corrective amplitude adjustment toeach transducer element in the phased array of ultrasound transducerelements based on the determined aberration for each respective element.2. The system of claim 1 wherein the unaberrated pressure field matrixcomprises a complex sum of the pressure field generated by eachtransducer element in the phased array of ultrasound transducerelements.
 3. The system of claim 1 wherein the aberration for eachrespective transducer element comprises a complex-valued attenuation andphase aberration generated by a bone or other intervening materialpositioned between the respective transducer element and the targettissue.
 4. The system of claim 1 wherein the computer is programmed todetermine an aberration for each transducer element of the phased arrayof ultrasound transducer elements from the pressure field matrix and theMR-ARFI images via analysis of an entirety of each of the MR-ARFIimages.
 5. The system of claim 1 wherein, in determining the aberrationfor each transducer element of the phased array of ultrasound transducerelements, the computer is programmed to minimize a cost function tosolve for an aberration vector of determinant length, with theminimizing being performed according to a magnitude least-squaresregression.
 6. The system of claim 1 wherein, in applying the at leastone of corrective phase adjustment and corrective amplitude adjustment,the computer is further programmed to: multiply an input signal of eachtransducer element by a respective aberration vector to impart bothamplitude and phase correction; or add an angle of each aberrationvector to a phase of its respective transducer element input signal. 7.The system of claim 1 wherein, in calculating a pressure field for eachtransducer element in the phased array of ultrasound transducer elementsto generate an unaberrated pressure field matrix, the computer isfurther programmed to generate the unaberrated pressure field matrixusing MR-ARFI images of a phantom or tissue sample, or using numericalsimulations in various tissue types.
 8. The system of claim 1 whereinthe computer is programmed to cause the gradient coils and the RF coilassembly in the MRI system to apply a fast spin echo (FSE) pulsesequence comprising a preparation segment and a plurality of refocusingsegments, wherein a pair of echoes is generated in each of the pluralityof refocusing segments that comprises: a first echo generated bymagnetization pathways having an even number of phase inversions; and asecond echo generated by magnetization pathways having an even number ofphase inversions.
 9. The system of claim 8 wherein the computer isprogrammed to: separate the first echo from the second echo; calculate aphase difference between the separated first and second echoes; anddetermine a phase accumulated during the preparation segment and a phaseinduced during the preparation segment by the MR-ARFI image acquisitionbased on the calculated phase difference between the first echo and thesecond echo.
 10. The system of claim 9 wherein, in separating the firstecho and the second echo, the computer is programmed to run two shots,with RF pulses in respective preparation segments of the two shotshaving different phases.
 11. The system of claim 9 wherein, inseparating the first echo and the second echo, the computer isprogrammed to apply a readout gradient pulse to the preparation segmentso as to shift the first echo in each refocusing segment, such that thefirst and second echoes are sampled in different sampling windows. 12.The system of claim 8 wherein the computer is programmed to: apply atwo-shot single-shot fast spin echo (two-shot ssFSE) sequence to acquirethe first and second echoes, wherein a first shot of the two-shot ssFSEsequence acquires odd k-space lines using an RF pulse in the preparationsegment having a first phase and wherein a second shot of the two-shotssFSE acquires even k-space lines using an RF pulse in the preparationsegment having a second phase different from the first phase; and employa parallel imaging technique with the two-shot ssFSE sequence, such thatimages of the even and odd k-space lines can be fully reconstructed withthe two-shot ssFSE sequence.
 13. A method for focusing a phased array ofultrasound transducer elements into a target tissue using MR-ARFI, themethod comprising: calculating a pressure field for each transducerelement in the phased array of ultrasound transducer elements togenerate an unaberrated pressure field matrix; causing the phased arrayof ultrasound transducer elements to apply focused ultrasound to thetarget tissue, wherein application of focused ultrasound by eachtransducer element of the phased array of ultrasound transducer elementsgenerates a respective aberrated pressure field at a focus location inthe target tissue and in a volume surrounding the focus location;acquiring magnetic resonance acoustic radiation force impulse (MR-ARFI)images of the focus location and the surrounding volume; based on theMR-ARFI images, measuring a magnitude of the aberrated pressure fieldgenerated by the applied focused ultrasound from each transducer elementof the phased array of ultrasound transducer elements at its respectivefocus location in the target tissue and in the volume surrounding thefocus location; determining an aberration for each transducer element ofthe phased array of ultrasound transducer elements from the pressurefield matrix and the MR-ARFI images of the focus location and thesurrounding volume using a magnitude least-squares method; and applyingat least one of corrective phase adjustment and a corrective amplitudeadjustment to each transducer element in the phased array of ultrasoundtransducer elements based on the determined aberration for eachrespective element.
 14. The method of claim 13 wherein the unaberratedpressure field matrix comprises a complex sum of the pressure fieldgenerated by each transducer element in the phased array of ultrasoundtransducer elements, and wherein the aberration for each respectivetransducer element comprises a complex-valued attenuation and phaseaberration generated by a bone or other intervening material positionedbetween the respective transducer element and the target tissue.
 15. Themethod of claim 13 wherein determining the aberration for eachtransducer element comprises determining an aberration for eachtransducer element from the pressure field matrix and from analysis of aplurality of voxels in the MR-ARFI images corresponding to the focuslocation and the surrounding volume.
 16. The method of claim 13 whereindetermining the aberration for each transducer element comprisesminimizing a cost function to solve for an aberration vector.
 17. Themethod of claim 16 wherein applying the at least one of corrective phaseadjustment and corrective amplitude adjustment further comprises:multiplying an input signal of each transducer element by its respectiveaberration vector to impart both amplitude and phase correction; oradding an angle of each aberration vector to a phase of its respectivetransducer element input signal.
 18. The method of claim 13 whereinacquiring MR-ARFI images further comprises: applying a fast spin echo(FSE) pulse sequence comprising a preparation segment and a plurality ofrefocusing segments; acquiring MR-ARFI image data from a pair of echoesin each of the plurality of refocusing segments, the pair of echoescomprising: a first echo generated by magnetization pathways having aneven number of phase inversions; and a second echo generated bymagnetization pathways having an even number of phase inversions.
 19. Anon-transitory computer readable storage medium having stored thereon acomputer program comprising instructions that, when executed by acomputer, cause the computer to: calculate a pressure field for each ofa plurality of transducer elements in an ultrasound transducer togenerate an unaberrated pressure field matrix, the ultrasound transducerconfigured to apply focused ultrasound to a target tissue; cause each ofthe plurality of transducer elements to apply focused ultrasound to thetarget tissue, with the application of focused ultrasound by each of theplurality of transducer elements generating a respective aberratedpressure field at a focus location in the target tissue and in a volumesurrounding the focus location; acquire one or more magnetic resonanceacoustic radiation force impulse (MR-ARFI) images of the focus locationand the surrounding volume, each of the one or more MR-ARFI imagescomprising one or more voxels corresponding to the focus location andvoxels corresponding to a region surrounding the focus location; measurea relative tissue displacement that each transducer element produces inthe target tissue responsive to the applied focused ultrasound, therelative tissue displacement produced by each transducer element beingmeasured using the voxels in the one or more MR-ARFI imagescorresponding to the focus location and the region surrounding the focuslocation; and calculate corrective phase and amplitude adjustments foreach of the plurality of transducer elements based on the measuredrelative tissue displacement.
 20. The non-transitory computer readablestorage medium of claim 19 wherein the instructions further cause thecomputer to: based on the one or more acquired MR-ARFI images, measure amagnitude of the aberrated pressure field generated by the appliedfocused ultrasound from each of the plurality of transducer elements;employ a magnitude least-squares method to determine an aberration foreach of the plurality of transducer elements from the pressure fieldmatrix and the one or more MR-ARFI images; and apply the correctivephase and amplitude adjustments to each of the plurality of transducerelements based on the determined aberration for each respectivetransducer element.